The invention relates in general to digital data storage devices and, more particularly, to digital data storage devices that utilize a head to transfer data from/to a moving medium.
A disk drive is a digital data storage device that stores data in substantially concentric tracks on the surface of a disk. During the operation of a disk drive, the disk is rotated at a substantially constant rate while data is written to or read from its surface. The disk drive is generally coupled to a host computer that delivers access requests to the disk drive whenever the host desires to store or retrieve digital information. To perform an access request, the disk drive first positions a head above the track of the rotating disk specified in the access request. Once the head is properly positioned, the requested data transfer (i.e., either a read operation or a write operation) is allowed to take place. During a read operation; data from the predetermined track is sensed by the head, after which it is processed by a read channel and delivered to the host system. During a write operation, data is received from the host, processed into a suitable format, and then delivered to the head which transfers the data to the predetermined track.
During operation of the disk drive, the head generally rides above the disk surface on a cushion of air (known as an air bearing) that is created by the movement of the disk under the head. The distance of the head from the disk while riding on the air bearing is referred to as the xe2x80x9cflying heightxe2x80x9d of the head. To produce the xe2x80x9cliftxe2x80x9d required to hold up the head, head xe2x80x9cslidersxe2x80x9d are generally used that have the requisite aerodynamic qualities. In general, the performance of the disk drive will depend, to a great extent, on the maintenance of a proper head flying height. That is, performance will be degraded if the actual flying height of the head is considerably higher, for example, than a nominal flying height. This is particularly the case during write operations where an unexpected increase in flying height can result in written data that is unreadable.
A need therefore exists for a method and apparatus for determining whether the actual flying height of a head is within a desired range.
The present invention relates to a system for determining whether the actual flying height of a head is within a desired range. The system is capable of operating xe2x80x9con the flyxe2x80x9d and may therefore be implemented in a disk drive system without substantially increasing disk access times. That is, the flying height determination can be made substantially in real time, before the head reaches the data sector to be written to or read from. This allows the drive to forego the transfer of data between the head and the disk when it is found that the head is outside of the desired flying height range, which is generally determined by empirical methods. The present invention has application in virtually any type of data recording system using either a contact, a pseudo contact, or a non-contact head to transfer data from/to a moving medium. This can include, for example, magnetic disk drive systems, magnetic tape systems, and optical disk drive systems. In one application, the invention is used in a system utilizing low flying height heads, such as the Tripad ((trademark)) head manufactured by Read-Rite.
To operate in real time, the invention relies on the relationship between read signal resolution and flying height. Read signal resolution is a performance measurement that is related to the disk drive""s ability to read information at different frequencies. In this regard, read signal resolution is generally calculated using the ratio of the magnitudes of two analog read signal portions having different frequencies. In conceiving of the present invention, it was appreciated that the ability of a read head to read data patterns at higher frequencies diminishes at a faster rate than the ability of the head to read lower frequency patterns as the head moves away from the disk. Because of this, read signal resolution changes in a predictable manner as the flying height of the head increases. In general, the invention determines a read signal resolution value, or a read signal resolution related value, and then compares the value to a predetermined value to determine whether the flying height is in the proper range.
In one aspect of the present invention, a disk drive is provided that comprises a disk having a first data pattern with a first frequency and a second data pattern with a second, higher frequency on a first track. The disk drive also includes means for reading the first and second data patterns, using a head at a first vertical distance from the disk, to create first and second analog signal portions, respectively. In addition, the disk drive includes a determination unit for determining whether the first vertical distance of the head is within an acceptable range for performing a transfer of user data between the first track and an exterior environment using the first analog signal portion and the second analog signal portion, wherein the determination unit does not require the movement of the head to a substantially different vertical distance to make the determination.
The determination unit determines whether the first vertical distance is within the desired range based on read signal resolution. The read signal resolution can be calculated by taking the ratio of the magnitudes of the first and second analog signal portions. The determination unit can include a comparison unit for comparing the calculated read signal resolution to a threshold resolution value that represents the maximum flying height that will result in an acceptable read or write performance. The resolution value can be stored in a memory along with other resolution values for different areas of the disk surface. For example, one stored threshold value can correspond to each zone on the disk.
The first and second data patterns can be stored anywhere within the first track. In one embodiment, the patterns are stored in a servo data region of the first track. To decrease overhead on the disk, the patterns can be stored in standard servo fields, such as the automatic gain control (AGC) field and/or the C/D servo burst fields. Alternatively, a dedicated servo field can be created for one or both of the patterns.
The disk drive can also include a unit for postponing the transfer of user data to the first track when it is determined that the head is not within the proper vertical distance range. For example, the first data pattern and the second data pattern can both be located in a servo sector immediately preceding a data area on the first track. The patterns can be read by the reading device and then the determination unit can determine whether the head is in the proper range. If the head is not in the proper range, the postponement unit can decide not to read or write data from/to the data area as the head proceeds to pass over the data area from the servo sector. The postponement unit can then perform a retry on the next revolution of the disk.
In one embodiment, the determination unit includes a transition detector for detecting indicia (such as, for example, peaks) in the second analog signal portion that are indicative of magnetic transitions stored on the surface of the disk. Because of the read signal resolution effect, when a relatively high frequency pattern is read by the head at an elevated flying height, some of the resulting indicia will not be detectable by the transition detector. Therefore, the number of indicia detected (and/or not detected) by the transition detector can be used as an indicator of flying height. The transition detector can include, for example, an analog peak detector, a PRML channel, a decision feedback equalizer, a finite delay tree search unit, or any other means for detecting indicia in a read signal.
In another aspect of the present invention, a disk drive is provided that includes a unit for determining whether the head is currently within an acceptable vertical distance from the disk surface for performing a transfer of user data between the predetermined track and the exterior environment before a transfer of user data is allowed to occur, wherein the determination does not include means for changing a current vertical distance between the head and the disk surface, and a transfer unit for performing the transfer of user data only when the head is determined to be within the acceptable vertical distance by the determination unit.
The determination unit operates in substantially real time so that the required determination can be made before the transducer reaches the data area to/from which data is to be transferred. The determination unit can make the required determination based upon read signal resolution or any other method capable of real time operation. In addition, the determination unit can include a comparison unit or peak detection unit as described above.
In yet another aspect of the present invention, a disk drive is provided comprising: a disk having a plurality of concentric tracks, the disk including a first pattern having a first frequency and a second pattern having a second frequency, the second frequency being greater than the first frequency; a head for use in transferring data to/from the disk; means for reading the first pattern using the head to create a first analog waveform having a first magnitude value; means for reading the second pattern using the head to create a second analog waveform having a second magnitude value; a combining unit for combining the first magnitude value and the second magnitude value to create a read signal resolution related value; and a comparison unit for comparing the read signal resolution related value to a threshold value.
In one embodiment, the threshold value is based on a minimum read signal resolution that will produce an acceptable data transfer performance in the disk drive. The combining unit can include a means for finding a ratio between the first magnitude and the second magnitude. The comparison unit can include a storage unit for storing a plurality of threshold values corresponding to different areas on the disk surface and a retrieval unit for retrieving one of the threshold values corresponding to a track currently being accessed.
In still another aspect of the present invention, a disk drive is provided comprising: a disk having a plurality of concentric tracks, the disk including a first track having a first data pattern with a first frequency; a head for use in transferring data to/from the disk; means for reading the first data pattern, using the head, to produce an analog read signal having a predetermined number of peaks representative of data on the disk surface; a processing unit for processing the analog read signal to determine which of the peaks in the analog read signal meet a predetermined detection criterion, wherein less than all of the predetermined number of peaks will meet the predetermined detection criterion when the head is not within the desired flying height range due to relatively low read signal resolution at the first frequency, the processing unit creating an output signal; and a determination unit for determining whether the flying height of the head is within the desired range based on the output signal of the processing unit.
In one embodiment, the processing unit determines how many of the peaks in the analog read signal meet the predetermined detection criterion and then compares the resulting peak count value to a threshold count value. As described above, the comparison unit may include a data storage unit for storing at least one, and preferably a plurality, of threshold count values. In addition, the determination unit can include a comparison unit for comparing the peak count value to a table of predetermined count values each having an associated flying height value to determine an actual present flying height value. The first data pattern can include, for example, a constant frequency data pattern or a variable frequency data pattern, such as a chirp or random pattern, that includes the first frequency. In addition, the processing unit may include a transition detector as described above.
In a further aspect of the present invention, a disk drive is provided that includes a read channel including a determination unit for determining whether a head is within an acceptable vertical distance from a disk surface for performing a transfer of user data between a track of the disk and an exterior environment. In one embodiment, the channel, including the determination unit, is implemented on a single semiconductor chip. In another embodiment, the channel includes an optimization unit for optimizing the channel for performing the determination function by changing the value of at least one of the channel parameters during a period when the determination unit is performing the determination function.